As a conventional vehicle information projection system which warns a user about an obstacle approaching a host vehicle, a head-up display (HUD) device as disclosed in Patent Literature 1 is known. Such a HUD device displays a relative distance between the host vehicle and a rearward vehicle (the obstacle) located on the rear of the host vehicle as a virtual image, whereby a user can view the existence of the rearward vehicle approaching the rear of the host vehicle and the relative distance together with outside scenery in front thereof.